This research is intended to contribute to our understanding of the chemical foundations of site-specific enzyme-DNA interactions. The EcoRI and RsrI restriction endonuclease and modification methyltransferase that catalyze two identical pairs of reactions at the same DNA sequence, viz., phosphodiester bond cleavage and adenine methylation, respectively, with the duplex DNA sequence GAATTC will serve as models. Specifically, the research will use an integrated biochemical, genetic, and structural approach to expand our understanding of the number and nature of the atomic interactions taking place between the enzymes and their DNA target during the course of the reactions in order to understand the exquisite specificities they display. Another facet of the work will use a genetic technique to select restriction enzymes and methyltransferases with altered DNA-sequence recognition specificities. Any such enzymes would be informative of basic mechanisms and could serve as useful tools in research and biotechnological applications. An additional aim of the research is to find new restriction endonucleases and methyltransferases that catalyze the same reactions as the primary set of enzymes. Comparative biochemistry will be used to discern the essential from the incidental attributes of the reactions. An important aspect of this work is that enzyme-DNA interactions and not the interactions between proteins that only bind site-specifically to DNA will be studied. Thus, the relative contributions of precatalytic binding and of catalytic specificity will be studied. The biochemical and genetic information engendered by this research will be supplemented with structural information derived from X-ray diffraction analyses to derive structure-function relationships. Any knowledge gained concerning the mechanism of action of the restriction and modification enzymes will contribute to our understanding of enzyme catalysis. These studies should help us better understand the chemical principles underlying the fundamental biological phenomenon of site-specific DNA interaction. Such knowledge is essential to any rational approach to the solution of the medical problems associated, either in etiology or therapy, with the synthesis, maintenance, alteration, function or inactivation of DNA. Cancer, bacterial and viral infections, and numerous metabolic disorders are intimately related to DNA-protein interactions. Ultimately, this research should contribute to the solution of these medical problems and to the understanding of the underlying biological phenomena.